For aluminium electrolytic coloration processes to be carried out to full satisfaction, a very thorough control on the current applied must exist.
Thus, for instance, Spanish patent of invention no. 498,578 and its U.S. Pat. No. 4,421,610, sets forth an electrolytic coloration process for an aluminium or aluminium alloy element, consisting of a first phase where, inter alia, an alternating current with a peak voltage lying between 25 and 85 volts and a current density below 0.3 amps. per square decimeter must be applied.
More specifically, and in order to obtain such alternating current, a polyphasic network or the secondaries in a polyphasic network transformer are used conducting the positive and negative half-cycles with the same conduction angle and both variables as required, which conduction angles are in turn controlled by reverse shunt thyristors or by triacs.
Said control of the thyristors' conduction angle obviously allows the average voltage to be controlled, but not so the peak voltage, and therefore the results attained, though acceptable, cannot be deemed to be the most favourable.
Manifold solutions have been put forward so far as electrolytic coloration processes are concerned, and the essential problem common to all is the difficulty of suitably controlling the currents applied to the vat.
Furthermore, from the theoretical viewpoint , opacification processes are known to attain, likewise by electrolytic processes, a transformation of the anodic film rendering the same opaque, but such processes require very low voltages in practice, less than three volts, and moreover very specific values, and no current control means exists presently that may allow the same to be maintained within the limits the process requires.
Optical interference aluminum coloration processes are also known, where the above-mentioned problem is even worse, for within a given range of voltages, minor variations in the value of the voltage lead to significant changes in the colour obtained, for which reason this system has not been developed industrially either, for the different load characteristics and the actual installation determine variations in the voltage drop and, hence, variations in the voltage applied to the load, originating undesirable colour changes.
There is hence no doubt whatsoever that the fact that there are presently no suitable means for controlling the current applied to electrolytic processes significantly constrains progress in this field.
In order to grasp the difficulties of the different aluminum electrolytic coloration systems it is worthwhile to note some of the phenomena that take place when applying an alternating current to the previously anodized aluminum:
During the positive half-cycle there is no deposition whatsoever at the anodic film pores. In the event of the voltage applied allowing passage of current, oxidation takes place, leading to an increase in film barrier thickness. The final film barrier thickness is proportional to the peak voltage applied.
During the negative half-cycle there is a double deposition. On the other hand, deposition of the metallic cation present in the form of a metallic particle. For instance: EQU Sn.sup.2 +2e.sup.- --Sn
Furthermore, deposition of protons present in the electrolyte, that become atomic hydrogen: EQU H.sup.+ +1e.sup.- --H
The speed of migration of the protons toward the bottom of the pores depends upon the voltage applied and the density of the circulating current. This latter in turn depends upon the total circuit impedance (see electric model of the U.S. Pat. No. 4,421,602, namely FIG. 1 thereof).
Because of the semiconducting nature of the film barrier, atomic hydrogen can be formed at low voltages, for instance at roughly 2 to 4 V. As higher voltages are applied and current circulation rises, this hydrogen can act differently: EQU GH+Al.sub.2 O.sub.3 --2Al.sup.3+ +3H.sub.2 O a) EQU H+Sn.sup.2+ --Sn+2H.sup.+ b) EQU H+H--H.sub.2 c)
Reaction a) takes place at voltages under 7-8 V.
Reactions b) and c) take place at voltages in excess of 8 V.
When the kinetic energy of the protons is very high, or film barrier resistance is weak, the protons can cross the film barrier and reaction c) can take place at the metal-oxide interface. In such event, the pressure generated by the accumulation of the molecular hydrogen formed can cause spalling.
These three types of effects caused by hydrogen can be regulated by accurately controlling the voltage applied during the negative half-cycle. The voltage in the positive half-cycle must be adjusted simultaneously to keep the circuit's impedance under control.
Thus:
With a), the bottom of the pores can be modified to cause the film barrier to become opaque, or the film barrier diameter and thickness adjusted in order to subsequently obtain the optical interference colours.
With b), the formation of metallic particles at the bottom of the pores can be enhanced; cations, for instance Sn.sup.2+.
Effect c) can be regulated by the separate positive half-cycle voltage control, that allows film barrier thickness to be increased, thereby to increase resistance and prevent spalling.
By analyzing these three effects, it can be clearly inferred that it is necessary to regulate and control the positive and negative half-cycle voltages and currents separately.
In electrolytic coloration processes, the passage of current is usually controlled and regulated indirectly by adjusting and controlling the voltage applied to the electric circuit (see FIG. 1 in U.S. Pat. No. 4,421,610). This adjustment is made through programs that linearly modify the voltage according to time.
The voltage must be modified as circuit impedance changes. If circuit impedance variation is not linear, neither can voltage variation be so. Thus, certain mathematical algorithms similar to those relating circuit impedance variations during the process must be applied at the voltage adjustment programs.